1. Field of the Invention
The invention relates to a solid-state optical device and, particularly, to a solid-state optical device with a sealing material of low-melting glass. Herein, the solid-state optical device includes various optical devices that are composed of a solid-state element (or a semiconductor element) such as a light emitting element (or LED element), a light receiving element and a solar cell element.
2. Description of the Related Art
A solid-state optical device is conventionally known in which a solid-state component such as an LED element is sealed with a transparent resin material such as epoxy rein. It is known that transparent resins deteriorate due to irradiation of light. Especially, when a group III nitride-based compound semiconductor LED element to emit a short-wavelength light is used, the transparent resin around the LED element will turn yellow due to a high-energy light emitted from the LED element and heat generated from the LED element. Therefore, its light extraction efficiency may lower significantly.
To prevent the deterioration of the sealing material, some solid-state optical devices are proposed (for example, JP-A-8-102553 and JP-A-11-177129).
JP-A-8-102553 discloses a light emitting device that an LED element, bonding wires and upper part of leads are sealed with a transparent sealing material of low-melting glass. The low-melting glass is, for example, glass to which selenium, thallium, arsenic, sulfur etc. are added to have a melting point of 130 to 350° C. In this case, it is preferable to use low-melting glass with a melting point of 200° C. or less, more preferably 150° C. or less.
The light emitting device of JP-A-8-102553 can solve the problem that the transparent resin material optically deteriorates with time due to the irradiation of UV light, by using the transparent sealing material of the low-melting glass.
On the other hand, JP-A-11-177129 discloses a light emitting device that an LED element is sealed with a sealing material of low-melting glass with a refractive index of about 2 which is close to 2.3, the refractive index of a GaN-based LED element.
The light emitting device of JP-A-11-177129 has a function that light totally reflected at the interface between the LED element and the low-melting glass is reduced by sealing the LED element with the low-melting glass with the refractive index close to that of the GaN-based LED element, and the amount of light to get into the low-melting glass while being emitted from the LED element is increased. As a result, the light extraction efficiency thereof can be increased as compared to the conventional LED element sealed with epoxy resin.
However, the abovementioned solid-state optical devices cannot have sufficient sealing properties even when the sealing material of low-melting glass is used since the conventional low-melting glass has a high viscosity in a working sealing temperature range.